Leptorhynchos gaddisi

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Leptorhynchos
Temporal range: Late Cretaceous[1]
~80.5 to 72 Ma - Campanian
Life reconstruction of L. gaddisi by one of the authors of its description
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Clade: †Oviraptorosauria
Superfamily: †Caenagnathoidea
Family: †Caenagnathidae
Genus: †Leptorhynchos
Longrich et al., 2013
Type species
†Leptorhynchos gaddisi
Longrich et al., 2013
Other species
Not to be confused with Leptorhynchos (plant).

Leptorhynchos (

US state of Texas, although it has been suggested to also exist in Alberta and South Dakota. The type species is L. gaddisi, and it is currently the only widely accepted valid species. The generic name of Leptorhynchos comes from the Greek "leptos" meaning "small" and "rhynchos" meaning "beak". The specific epithet is in honor of the Gaddis family, who owned the land on which the holotype was discovered.[2]

Discovery

A picture of an outcrop of the Aguja Formation at Big Bend National Park

Leptorhynchos was named in 2013 by Nick Longrich, Ken Barnes, Scott Clark, and Larry Millar. In their description, they conducted a specimen-level review of North American caenagnathids in an attempt to resolve the perennial taxonomic confusion surrounding the group. This included the type specimens of Caenagnathus, Chirostenotes, and Hagryphus as well as specimens of more ambiguous affinities such as ROM 781, which was previously named as a species of Ornithomimus. Their analysis also included the specimen TMM 45920-1, which would later be designated as the holotype of Leptorhynchos gaddisi.[2][3]

The results of Longrich and colleagues' analysis was the naming of the new genus, Leptorhynchos, and the assignment of "Ornithomimus" elegans (specimen ROM 781) as a second species in that genus. Several other specimens, such as MOR 1107, were referred to the genus but not assigned to a particular species. Their description contained an initial and then a revised diagnosis of the genus and both species.[2][3] Remains of other small caenagnathids from Laramidia discovered in the Hell Creek and Scollard formations have variously been attributed to the genus, but without being assigned to either of the named species.

Description

Species

Leptorhynchos gaddisi

L. gaddisi was named as the type species of the new genus by Longrich and colleagues. They based the diagnosis of this species on the anteriorly-projected beak tip, rounded ventral margin of the

dentary symphysis, and a spoon-shape to the beak tip in dorsal view.[3] Longrich and colleagues did not provide a mass estimate in their description but Rubén Molina-Pérez and Asier Larramendi suggested a length of 1.75 metres (5.7 ft) and a mass of roughly 19 kilograms (42 lb).[4] L. gaddisi is generally regarded as the only valid species of Leptorhynchos.[5]

"Leptorhynchos" elegans
See also: Citipes
The holotype material of Citipes (formerly Leptorhycnhos elegans)

The fossil specimen ROM 781 was described in 1933 as a new species of the genus

junior synonym of Chirostenotes), then to Elmisaurus, and then to Chirostenotes before being named by Longrich and colleagues as the type specimen of the new species L. elegans.[5] They made this diagnosis based on the strongly upturned beak and straight anterior margin of the dentary symphysis and also referred the specimen TMP 1992.36.390 to the species. However, in their phylogenetic analysis which accompanied the description, the authors remarked that the two species form a polytomy with one another within Caenagnathidae and they may not represent the same genus.[2] In 2020, Gregory Funston reassigned L. elegans to the new genus, Citipes.[5] Leptorhynchos has not been included in any phylogenetic analyses with Citipes since the most recent referral, so the veracity of this reassignment has yet to be tested by new authors in order to affirm or contest it.[6]

Leptorhynchos sp.
Caenagnathid bones from the Frenchman Formation which may have belonged to Leptorhynchos

In their description of the genus, Longrich and colleagues referred the specimen MOR 1107 (part of a

caenagnathid remains have been variously referred to the genus since it was named, although none have been named as a their own species. Among these are caenagnathid remains from the late Maastrichtian Scollard Formation,[7] the Frenchman Formation,[8] and the Hell Creek Formation.[9] Subsequent authors have disputed these referrals.[6] More caenagnathid remains have also recently been described from the Aguja Formation, which have been speculated to belong to Leptorhynchos.[10]

Skeleton

The

metatarsals, and a single toe claw. These specimens were not associated with the holotype, but were referred to L. gaddisi based on their size and discovery in the same geologic layer.[2] Later authors have questioned this referral[5] and Leptorhynchos has not been included in some recent analyses.[5][6]

The dentaries of the holotype were fully fused with no visible sutures, which suggest that the animal was fully mature when it died and the small size of the specimen is not the result of

caenagnathid.[2]

The referred specimens from the Aguja Formation are tentatively assigned to this taxon. They include a single partial vertebra, two partial metatarsals, and a pedal ungual. The vertebra is unremarkable with twin

tarsal bones, which is the condition seen in both Citipes and Elmisaurus.[2]

Classification

Diagram of most known caenagnathid remains

In their description of the holotype, Longrich and colleagues conducted a phylogenetic analysis. Their analysis used the dataset of Longrich's earlier publication in 2010 with several new taxa added for a total of 28 taxa coded for 205 characters. Their data set also included recent information about

surangular bone. The analysis did resolve several taxa at the base of caenagnathidae including Gigantoraptor and Microvenator, but there was less resolution in the more derived area of the tree. In particular, their strict consensus tree did not support the unambiguous monophyly of the genus Leptorhynchos; the two species they assigned to the genus were in a polytomy with Hagryphus and a clade containing Caenagnathus, Chirostenotes, and the specimen BMNH 2033 (which is from the Hell Creek Formation and was coded as a distinct taxon). The authors remark that they referred ROM 781 to Leptorhynchos because of the similar size of the two species, but noted that they may belong to separate genera, pending further research.[2]

Longrich and colleagues broadly determined that most

oviraptorosaurs, which can be used to determine if an individual specimen is a new taxon or simply a different life stage of an existing taxon.[2]

For specimens which do not contain the relevant diagnostic characters or any indication of their ontogenetic age, they have been assigned to existing taxa according to size and locality. Longrich and colleagues note that this approach has shortcomings, but they argue that the ecological abundance of adult animals in any ecosystem means that most fossilized animals will be adults anyways, which they suggest is sufficient for the purposes of their analysis. A consensus of the 116 most parsimonious trees in their analysis is shown below.[2]

Oviraptorosauria

The most recent

caenagnathid genera in their analysis. They recovered Leptorhynchos and Citipes (then L. elegans) in a polytomy with the genera Elmisaurus and Apatoraptor within a monophyletic "elmisaurinae". They do not comment any further on the implications of this analysis on caenagnathid phylogeny.[11] Subsequent analyses which have not included Leptorhynchos have not recovered these taxa as being closely related.[5]

Caenagnathoidea

Paleoecology

A map of known caenagnathid remains from North America
A reconstruction of a hadrosaur from the Kirtland Formation, which would have been climatically similar to the Aguja Formation

Paleoenvironment

The only definite remains of Leptorhynchos have been recovered from the Upper Shale Member of the

Cretaceous Period, this area would have been a freshwater or brackish coastal marsh and a floodplain that was part of a river delta.[14] The sediments also record the presence of oxbow lakes and some coastal marine deposits.[15]

Contemporary fauna

A wide variety of

freshwater ecosystem with very few marine animals.[16]

A wide variety of

varanoids, and snakes.[17] The giant alligatorid Deinosuchus riograndensis has also been found in the Aguja Formation, but is not known from the Upper Shale Member.[18]

Dinosaurs are the largest animals found in the Aguja Formation, and the taxa found in these rocks is broadly similar to contemporaneous faunal assemblages in the

osteoderms and vertebrae.[24] These have been referred to the genera Edmontonia, Euoplocephalus, and Panoplosaurus.[14]

caenagnathids were present in the region during the Late Cretaceous. A few specimens have been referred to the genera Chirostenotes, Saurornitholestes, Dromaeosaurus, Troodon, and the enigmatic Richardoestesia, but these referrals remain uncertain.[14][27]

See also

References

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  9. ^ Stein, Walter W. (2019). "TAKING COUNT: A Census of Dinosaur Fossils Recovered From the Hell Creek and Lance Formations (Maastrichtian)" (PDF). The Journal of Paleontological Sciences. 8: 1–42.
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  13. ^ Mannion, Philip (2013). "Terlingua area (Cretaceous of the United States); Also known as TMM 45920". The Paleobiology Database. Where: Brewster County, Texas
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  24. ^ West, Bryanna (2020). Campanian-Maastrictian Ankylosaurs of West Texas (PDF) (Thesis). Texas Tech University.
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  26. ^ Sullivan, R.M., and Lucas, S.G. 2006. "The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America[permanent dead link]." New Mexico Museum of Natural History and Science, Bulletin 35:7-29.
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